docs: Resolves Detail design choices in README
docs: Resolves Detail design choices in README

Add design choices for Speaker and Session. Add query problem explanation and solution.
Add additional queries endpoints and brief description.

Resolves #22

iraquitan/udacity-fsnd-p4-conference-app,iraquitan/udacity-fsnd-p4-conference-app,iraquitan/udacity-fsnd-p4-conference-app